Official Answer
Answer
Decarburization is the loss of carbon from the surface layer of the steel during heat treatment in an oxygen-rich atmosphere. This creates a soft 'ferrite' skin with a much lower yield strength than the core. For a wave spring, which relies on surface fiber stress $\sigma = \frac{3 \pi P D_m}{2 N^2 b t^2}$, decarburization leads to immediate 'setting' (permanent loss of free height) upon the first compression. Metallographic examination (per ASTM E1077) would reveal a lighter-colored surface layer. This is avoided by using 'atmosphere-controlled' furnaces or vacuum heat treatment for high-reliability parts.
